Microfacies d’une lentille biohermale a la limite Eifelien/Givetien (Wellin, bord sud du synclinorium de Dinant)

A biohermal lens is described from the Eifelian-Givetian transition beds near Wellin (southern flank of the Dinant basin). The lens is about 200 m long and 45 m thick. It is composed of a central stromatopore-coral framestone (30 m) and of flanks of udoteacean packstone and stromatopore-brachiopod floatstone. The sedimentology based on two nearby sections indicate a regression. The log is based on the succession of 10 carbonate microfacies (MF1-10, standard sequence). The deepest microfacies (MF1) is open marine at the upper limit of the storm waves and the dysphotic-euphotic boundary. The shallower microfacies is partly emerged (lagoonal sediments, MF10). The framestone (MF7) is characterized by early isopachous cementation. Communities and fossil assemblages underline the important development of udoteacean meadows in the flanks while these algae play little or no role in the formation of the framestone. The bioherm covers a crinoidal sole (MF4), that is overlying the open marine facies (MF1-3). These show evidence of sulfate-reduction, with widespread bacterial (?) filamentous pyrite. Correlation between the two stratigraphic sections indicates that the architecture of the bioherm (central core and flanks) is planar (‘bank »). The total thickness of the studied succession is about 70 m. They grade from dysphotic-euphotic boundary estimated here around 20 m deep to emersion. Thus subsidence is here much more important than the eustatic regression. A new green udoteacean alga is described : Vignella nilsii n.gen., n. sp

Evolution sédimentaire et Ostracodes de la base du Givetien à Resteigne (bord sud du Bassin de Dinant, Belgique)

The facies patterns of lower Givetian rocks (Trois-Fontaines, Terres d'Haurs and Mont d'Haurs Formations) at Resteigne quarry (southern border of Dinant Basin, Belgium) reveal a transition from mixed siliciclastic carbonate ramp to gently sloping carbonate platform without any true reefal barrier. Depositional conditions on the ramp were open marine, well circulated, well lit and very near or within wave base. The ramp evolved through a very shallow-subtidal to supratidal, restricted to semi-restricted, quiet stable platform during early Givetian times. A sedimentological model for this succession enables recognition of 12 major microfacies, ranging from open marine, below wave action zone, to reefal complexes, lagoons and supratidal mats. These microfacies define three principal facies belts. Four ecozones belonging to the "Eifeler ökotyp" are recognized in the Givetian of Resteigne quarry. Ecozone 0 is characterized by Leperditicopida in the middle and upper parts of the Trois-Fontaines Formation and indicates a restricted lagoonal environment; three ostracod sub-assemblages can be recorded in term of salinity. Ecozone I is characterized by a few species of Platycopina, Palaeocopina or Eridostraca, each present in large numbers; it indicates an open lagoonal environment in the Terre d'Haurs Formation and, exceptionally, in the middle and upper parts of the Trois-Fontaines Formation. Ecozone II, dominated by large, thick shelled Podocopina, indicates water turbulence at the base of the Trois-Fontaines Formation and in the Mont d'Haurs Formation. Ecozone III is rich in species of Podocopina, Platycopina, Metacopina, Palaeocopina and Eridostraca; it indicates a shallow open marine environment in the Mont d'Haurs Formation and, exceptionally, in the Terres d'Haurs Formation. Water movement and salinity are the principal ecological factors controlling the composition of the basal Givetian ostracod fauna at Resteigne. Their detailed analysis gives more information about the elementary regressive rhythms that produce cyclic patterns on the platform. A new species is erected: Evlanella lessensis n. sp

Sédimentologie et Ostracodes de la Limite Eifelien-Givetien à Resteigne (Bord Sud Du Bassin de Dinant, Belgique)

The global cyclic succession of the microfacies from the Upper Eifelian to Lower Givetian times shows evidence of shallowing from deeper settings, unaffected by storm-reworking, into «proximal» above storm wave base settings, near a shoreline coast. This overall regressive progradational megasequence indicates that deposition could have occurred on a ramp setting without any kind of continuous rims and-or typical lagoonal facies behind them. This general upward-coarsening progradational evolution corresponds probably to a phase of reduced sea-level rise or stillstand on the top of which the ramp setting has evolved into a carbonate progradational accretionnary platform. Forty-one ostracod species are reported and twenty-four described from the Hanonet Formation and from the base of the Trois-Fontaines Formation of the Resteigne quarry. Sixteen are known mainly from Germany and Poland and five of them are closely related to well established species. Sixteen species are placed in open nomenclature, three of which are new, and eight are comparable with previously described data. Aparchites resteignensis n.sp. is described in detail. The ostracod fauna indicates a shallow marine environment and an increase of water agitation near the Eifelian/Givetian boundary

Ostracods and lithofacies close to the Eifelian-Givetian boundary (Devonian) at Aisemont (Namur Synclinorium, Belgium)

The paper presents the sedimentological analysis and the ostracod study of the upper part of the Rivière Fm. and of the stratotype of the Névremont Fm. in the Aisemont section, the reference section for the Eifelian-Givetian boundary (Devonian) in the southern border of the Namur Synclinorium, Belgium.Thirteen major microfacies types are defined. Their stratigraphic succession records the transition from a mixed siliciclastic-carbonate ramp (upper Eifelian) to a carbonate platform (lower Givetian). About 6,350 ostracods were extracted. Forty species (19 in open nomenclature) are recognized in the Rivière Fm. and 48 species (27 in open nomenclature) in the Névremont Fm. They belong exclusively to the Eifelian Mega-Assemblage and their distribution is controlled mainly by water energy and salinity variations.The sedimentological analysis and the study of ostracods are in good agreement with recent conodont study of GOUWY & BULTYNCK (2003). Our work confirms the position of the Rivière and Névremont formations boundary and highlights a gap of the major part of the Hanonet and Trois-Fontaines formations of the southern Dinant Synclinorium. This hiatus is probably associated with the lowstand systems tract at the Eifelian-Givetian boundary interval in northern France and southern Belgium (PRÉAT, 2004).The presence of Quassilites fromelennensis and Jenningsina heddebauti, indicates that a great part of the transitional zone of GOUWY & BULTYNCK (2003) is of Givetian age

Givetian

The Givetian Stage in the Givet area is represented by the main part of the Hanonet Fm, the Trois-Fontaines Fm, the Terres d'Haurs Fm, the Mont d'Haurs Fm, the Fromelennes Fm and the lowest part of the Nismes Fm. During Eifelian-Givetian transition, the sedimentary systems evolved from a mixed siliciclastic-carbonate ramp to a carbonate platform dominated by cyclic tidal-flat and lagoonal wackestones with local patch reefs or coralgal banks. The shelf had an ESE-WNW trend and extended from the Avesnois basin (northern France) in the west to Aachen (western Germany). The sedimentation in the Ardennes consists of shallow water regressive metric (around 5 m or less) cycles. Stromatopores, corals, brachiopods, algae and cyanobacteria are abundant. Due to this general shallow water setting, the base of the Givetian Stage has been defined by the IUGS Subcommission on Devonian Stratigraphy in a GSSP in southern Morocco referring to the first occurence of the condodont species Polygnathus hemiansatus. The conodont species Icriodus obliquimarginatus appears approximately at the same level in Belgium

Ostracodes et sédimentologie du sommet de l'Eifelien et la Base du Givetien, à Couvin (bord sud du Bassin de Dinant, Belgique)

About seventy ostracod species are present in the upper part of the Hanonet Formation (Upper Eifelian) and in the base of the "Trois-Fontaines" Formation (Lower Givetian) of the "La Couvinoise" quarry at Couvin (southern border of Dinant Basin, Belgium). The ostracods belong to the "Eifeler Okotyp" and four associations are recognized which characterize successively a quiet open marine environment on the outer ramp (ecozone III), a shallow marine environment with increasing of water turbulence on the mid- and inner ramps (ecozone II), an open lagoonal environment on the upper part of the inner ramp (ecozone I) and a restricted lagoonal environment on the platform (ecozone 0). This sequence is the same as in the Resteigne quarry (50 km E of Couvin) studied by CASIER & PREAT (1990 & 1991) and is closely related to environmental variations. The principal ecological factors controlling the composition of the ostracod fauna are water movement in the Hanonet Formation (Eifelian), and water movement and salinity in the Trois- Fontaines Formation (Givetian). Two new species are erected: Condracypris? covinumensis n. sp. et Microcheilinella? hanonetensis n. sp

Ostracods, rock facies and magnetic susceptibility of the Hanonet Formation / Trois-Fontaines Formation boundary interval (Early Givetian) at the Mont d’Haurs (Givet, France)

Approximately 870 carapaces, valves and fragments of ostracods were extracted from 26 samples collected in the Hanonet Formation (= Fm) and Trois-Fontaines Fm in a section located along the southwestern rampart of the historically entrenched military camp at the Mont d’Haurs (Givet, France). Forty-eight species belonging to the Eifelian Mega-Assemblage have been identified and three new are proposed: Coryellina? audiarti nov. sp., Cavellina haursensis nov. sp. and Parabolbinella coeni nov. sp. The ostracod assemblages are generally indicative of shallow marine well-oxygenated environments, except at the base of the Trois-Fontaines Fm where ostracods indicative of semi-restricted and even of lagoonal water conditions are reported.The sedimentary record represents a transition from mixed siliciclastic-carbonate open-marine ramp system to restricted carbonate platform with deposition in low-energy peritidal and lagoonal settings frequently affected by subaerial exposition. A general trend follows a shallowing-upward and a relative sea-level decrease from the Hanonet Fm toward the Trois-Fontaines Fm.High-energy characterized the ramp setting and destroyed most of the stromatoporoid and coral buildups, which occur as floatstone and rudstone accumulations forming a thick biostrome. Girvanella and issinellid shoals developed also in this high-energy environment. The low-field magnetic susceptibility (=MS) log plotted against the lithological column reveals four magnetic sequences. The MS log and microfacies are moderately correlated mainly due to the evolution of sedimentary environments from a ramp to a platform. The MS values of the Trois-Fontaines Fm are very low in the biostromal unit whereas restricted lagoonal facies are characterized by the highest values of MS. A high-resolution stratigraphic correlative pattern is proposed here between the Mont d’Haurs section and the 40 km distant Baileux section despite the greater thickness. The MS signal is strongly controlled by minerals of ferromagnetic characteristics with a minor contribution of paramagnetic phases. The lagoonal sediments of the Trois-Fontaines Fm are characterized also by the highest values of normalized viscosity coefficient and a IRM loss. These results confirm the occurrence of a significantly high proportion of ultrafine magnetic grains which may be formed during diagenesis by chemical remanent magnetization processes.The list of the Givetian ostracods figured by Coen (1985) and recently lodged at the Department of Paleontology at the Royal Belgian Institute of Natural Sciences is reported in annex with new inventory numbers

Microfaciès d’une lentille biohermale a la limite Eifelien/Givetien (‘Fondry Des Chiens’, Nismes, bord sud du synclinorium de Dinant)

Microfacies of a biohermal lens at the Eifelian.Givetian transition (‘Fondry des Chiens’, Nismes, southern border of the Dinant Synclinorium). The biohermal lens of the ‘Fondry des Chiens’ belongs to the Eifelian-Givetian transition beds near Nismes (southern flank of the Dinant basin). The lens is 64 m thick and composed of a stromatopore-coral framestone. It is overlain by restricted lagoonal algal and cyanophycean facies near the emersion, and overlies Udoteacean and coral-bryozoan coverstones forming the flanks of two other unexposed lenses. Two crinoidal soles stabilized by syntaxial cementation constitute the substratum of these lenses. The reefal sedimentation is regressive. The log is based on the succession of 10 carbonate microfacies (MF1-10, standard sequence). The deepest microfacies (MF1) is open marine at the upper limit of the storm waves and the dysphotic-euphotic boundary. The shallowest sediments were partly emerged (lagoonal sediments, MF10). The exposed reefal lens (rudstones and framestones, MF6-7) and the flanks (grainstones, floatstones and coverstones, MF3-4-5) of the two other lenses are preserved due to early isopachous intergranular cement in the original cavities of the framestones or ‘intramicritic’ (replacement of the matrix) cementation in the floatstones and coverstones. The similarity of the facies and their algal content suggest that the sedimentary model proposed at Wellin is applicable at Nismes. Sequential analysis points to a three steps regressive evolution of the sedimentation probably related to a discontinuous subidence. The sequences have similar thicknesses (sixty or so meters) and grade from the dysphotic-euphotic boundary estimated here around twenty meters deep to emersion. As for Wellin, the subsidence is thus much more important than the eustatic regression

Sedimentologie paleoecologie et paleontologie des calcaires crinoidiques au voisinage de la limite couvinien-givetien a Wellin

The sedimentology of the Eifelian-Givetian boundary beds in the Wellin area indicates a transition from a siliciclastic-carbonate ramp to a gently sloping carbonate platform lacking a true reefal barrier. Prograding decametric sedimentary units form a littoral accretion megasequence. Corallian megafauna and algal microflora are highly diverse and abundant. Several communities are recognized in the ramp but they cannot be used for precise chronostratigraphic correlations.Comparison with other regions of the Dinant Synclinorium shows that the basin was already partitioned into blocks in Late Eifelian time. In Early Givetian several blocks of plurikilometric extension are identified. In this context, recognition of a new formation, provisionnally called "Formation X", between the Jemelle and Hanonet Formations, is significant.This local unit, some 120 meters thick, is composed of crinoidal and reefal limestones and indicates the existence of a sharply subsiding block in the Wellin area

Ostracods, rock facies and magnetic susceptibility of the Trois-Fontaines and Terres d’Haurs Formations (Early Givetian) in the Rancennes quarry at the Mont d’Haurs (Givet, France)

About 1,200 ostracods were extracted from 64 samples collected in the upper part of the Trois-Fontaines Formation (Fm) and in the base of the Terres d’Haurs Fm in the Rancennes quarry located along the western rampart of an historic entrenched military camp at the Mont d’Haurs (southern part of the Dinant Synclinorium, Ardennes Department, France). The ostracod richness and diversity are quite variable, and monospecificity prevails in numerous samples. Forty-nine ostracod species are recognised. In the Trois-Fontaines Fm, environments were lagoonal or semi-restricted, and the level containing numerous Leperditicopid ostracods (Herrmannina) indicative of (brackish?) lagoonal environments is 40 m thick. In the Terres d’Haurs Fm the environment was semi-restricted or more frequently shallow marine but the energy of the environment was apparently never very high. The level rich in Leperditicopida (Herrmannina) in the Trois-Fontaines Fm corresponds remarkably to the highest magnetic susceptibility (MS) value. The Rancennes microfacies point to a tidal flat system with various subenvironments such as restricted intertidal, supratidal and channel deposits. The system was bordered by subtidal open marine deposits where former reefal constructions have been destroyed. Frequent oscillations in this low-gradient shallow platform led to the exposure and modification of marginal ponds, floodplain environments or palustrine areas. No evidence of evaporitic environments or sabkha were encountered. The sedimentary system records the evolution of a shallow restricted carbonate platform (Trois-Fontaines Fm) to a carbonate ramp setting (Terres d’Haurs Fm). The evolution of the platfom to a ramp could be related to the cessation of the active role of a reefal barrier possibly as a response to synsedimentary tectonism and block faulting.The magnetic susceptibility curve established for the Rancennes quarry highlights 26 short-term magnetic evolutions that can be grouped into 10 major long-term magnetic sequences characterized by decreasing, increasing or stable magnetic susceptibility fluctuations. Magnetic susceptibility values range between 3.75 x 10-9 and 2.98 x 10-7 m³/kg. There is a general good correspondence between the microfacies and magnetic susceptibility curves, which are clearly mimetic at the smaller scale (i.e., 5th-order parasequences). The magnetic susceptibility curve could thus be interpreted as sea-level oscillations. A part of the magnetic minerals carrying the MS signal must have a detrital origin. Magnetization and coercivity ratios deduced from hysteresis loops indicate the presence of detrital coarse-grained multi-domain magnetite and authigenic mixtures of fine-grained superparamagnetic and single-domain magnetite. The MS signal of the Rancennes quarry seems to be controlled by the ferrimagnetic fraction (magnetite) with minor paramagnetic contribution (clay minerals and pyrite).The Rancennes quarry completes the stratotype of the Terres d’Haurs Fm because the section exposes the boundary with the Trois-Fontaines Fm unlike the previously proposed stratotype located on the south-eastern flank of the Mont d’Haurs entrenched camp